The present invention relates to an iron-based heat-resistant cast alloy. It relates more particularly to an iron-based heat-resistant cast alloy possessing enough high-temperature creep rupture strength and creep rupture ductility necessary for high-temperature, high-pressure piping material, and also having a sufficient weldability to be used as welded structure.
Prior to the present invention, various cast alloys such as HK40 (25Cr-20Ni steel) (see ASTM A608() and HU (19Cr-39Ni steel) (see ASTM A297) and forged alloys such as Incoloy 800 (see ASTM B407) have been used as the materials for manifolds and collectors of furnaces for producing hydrogen, methanol and ammonia, naphtha modifying furnace and other heat treating furnaces.
However, since, for example, the manifold of naphtha modifying furnace is exposed to high temperatures of 700.degree. to 900.degree. C. and high pressures of 10 to 30 atm., and yet to irregular cycles of thermal expansion and shrinkage, the conventional cast alloys and forged alloys were inadequate as the materials for the manifold.
This is because the cast alloys, such as said HK40 and Hu, produces, when heated to a high temperature, fine secondary carbides in their metal structure, which are hardened to deteriorate the toughness. On the other hand, in the case of Incoloy 800 forged alloy, since the creep rupture strength is low, a larger thickness is needed, as compared with cast alloy, under the identical operating conditions, which naturally results in a heavier weight. That is, since the wall thickness is larger, the temperature difference between the inside and outside of the wall becomes larger, which subsidiarily lead to thermal stress.
Accordingly, seeing that the manifold of naphtha modifying furnace is part of high-temperature, high-pressure piping line, and that the manifold itself is complicated in shape and is sure to be welded in the fabrication process thereof, the material for this manifold requires high creep rupture strength, excellent ductility, and good weldability. All these three conditions are not, in reality, satisfied in the conventional cast alloys and forged alloys.